This invention relates to a method for the preparation of unsaturated isocyanates.
It is well-known that carbamates can be thermally cracked or pyrolyzed, to form the corresponding isocyanates, i.e., isocyanates which are identical to the starting molecule except for the conversion of the carbamate moiety to an isocyanate moiety. For example, see U.S. Pat. Nos. 2,409,712 and 2,713,591. U.S. Pat. No. 4,386,033 teaches the production of monoisocyanates from monocarbamates using an alcohol having a boiling point which is at least .+-.50.degree. C. from the boiling point of the isocyanate produced. U.S. Pat. No. 3,919,279 discloses a method for producing saturated or unsaturated mono- or polyisocyanates via contacting the corresponding mono- or polycarbamates in certain solvents in the presence of a heavy metal at elevated temperatures. Dicarbamates yield diisocyanates using said method. Similarly, unsaturated carbamates yield unsaturated isocyanates via that method. U.S. Pat. No. 4,294,774 discloses a process for the preparation of an organic isocyanate by using an N,N-dialkyl aniline compound as a selectively-catalytic solvent. These patents are merely examples of known methods for the production of the corresponding isocyanate from the carbamate.
European patent application No. 55,359 teaches cleavage, at elevated temperature, of unsaturated monocarbamates to obtain the corresponding 1-alkenyl isocyanates. U.S. Pat. No. 4,388,246 discloses a similar thermal splitting reaction for di- and polycarbamates.
Heretofore, a method for producing an unsaturated monoisocyanate from a saturated geminal dicarbamate in a single step has not been disclosed.